Method for preparing corrosion-resistant ceramic shapes

Abstract

Ceramic shapes having impermeable tungsten coatings can be used for containing highly corrosive molten alloys and salts. The shapes are prepared by coating damp green ceramic shapes containing a small amount of yttria with a tungsten coating slip which has been adjusted to match the shrinkage rate of the green ceramic and which will fire to a theoretical density of at least 80% to provide a impermeable coating.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A method of preparing a refractory ceramic shape having an impermeable and corrosion-resistant metallic tungsten coating comprising: preparing a ceramic casting slip consisting of a finely divided ceramic powder, 0.5 to 5.0 weight percent yttria powder and a suspending agent in a water vehicle, the particle size of the ceramic powder being such that a shape formed from the slip will fire to a density of at least 90% of theoretical and the shape will have a predetermined shrinkage rate during firing,   forming a green ceramic shape from the slip,   drying the green shape until it contains about 5 to 12 weight percent moisture,   preparing a coating slip consisting of finely divided tungsten powder and about 0.5 to 1.0 weight percent ammonium alginate as a suspending agent in a water vehicle, the particle size of the tungsten powder being such that the tungsten slip will fire to a density of at least 80% of theoretical and will have a shrinkage rate during firing which is about the same as the shrinkage rate of the ceramic slip,   applying a coating of the tungsten slip to the surface of the damp green ceramic shape to form a green composite shape, and   firing the green composite shape in a vacuum or reducing atmosphere at a temperature and for a period of time sufficient to sinter and densify the green ceramic and the tungsten coating, thereby forming a dense ceramic shape having an impermeable and corrosion-resistant metallic tungsten coating.   
     
     
       2. The method of claim 1 wherein the ceramic material is alumina and the coating of tungsten slip is at least 20 μm thick. 
     
     
       3. The method of claim 1 wherein the ceramic slip contains from about 0.4 to 0.6 weight percent ammonium alginate as a suspending agent. 
     
     
       4. The method of claim 2 wherein the coating of tungsten slip on the surface of the damp green shape is from 60 to 150 μm in thickness. 
     
     
       5. The method of claim 3 wherein the ceramic slip contains from about 60 to 65 percent of alumina and the tungsten coating slip contains from about 75 to 80 weight percent tungsten powder. 
     
     
       6. The method of claim 4 wherein the green composite ceramic shape is fired to about 1800° C. for not less than one hour.